1. Field of the Invention
The present invention concerns a gas pressure regulator particularly suited to be used in a natural gas distribution network.
The present invention concerns also a method for piloting said pressure regulator.
2. Present State of the Art
As is known, natural gas distribution networks comprise pressure regulation stations, in which the gas pressure is reduced from the feeding value to the delivery value required by the user unit.
Said pressure reduction is obtained by means of pressure regulators that have the function to keep the delivery pressure at a predefined setting value, independently of the flow rate of the delivered natural gas.
A pressure regulator of the known type comprises a natural gas flow duct having one end that communicates with the high-pressure branch of the distribution network and the opposite end that communicates with the low-pressure branch directed towards the user unit.
In the flow duct there is a shutter that defines a narrowing in the flow duct itself, so as to cause a gas pressure drop between the pressure downstream of the shutter and the pressure upstream of the shutter.
The shutter can be moved so that the cross section of the narrowing and, therefore, the gas pressure drop can be modified according to the gas flow rate.
The shutter is moved by means of a feedback circuit that, in the presence of a pressure increase in the delivered gas with respect to the value set in the regulator, reduces the extent of opening of the shutter, and vice versa in the case of a pressure decrease.
Said feedback is based on the use of a motorisation chamber, which is delimited by a movable wall connected to the shutter and associated with a spring.
As long as the pressure in the motorisation chamber is balanced with the force of the spring, the movable wall remains stationary.
In the presence of a pressure perturbation, the movable wall moves until a balanced condition is restored, causing a corresponding movement of the shutter.
According to a first known technique, the motorisation chamber is placed directly in communication with the flow duct in an area downstream of the shutter, so that the movable wall is subjected to the delivery pressure of the natural gas.
The movable wall is connected to the shutter in such a way that a reduction in the delivery pressure following, for example, a flow rate increase, causes the shutter to open so as to restore the initial pressure.
The regulator just described above is called “direct action” regulator, as the movable wall that controls the shutter is directly affected by the delivery pressure of the natural gas.
In this case, the set pressure is regulated by intervening on the spring preload.
Obviously, a direct action regulator responds very quickly to pressure variations, given that the delay is only the delay related to the mechanical inertia of the components of the regulator and to the time necessary for the propagation of the pressure perturbation from the flow duct to the motorization chamber.
On the other hand, said direct action regulator offers a relatively limited regulation precision, deriving from the fact that the force of the spring is not uniform as its deformation varies.
Consequently, the gas pressure that is necessary to balance the force of the spring depends on the deformation of the latter and, therefore, on the position of the movable wall.
Therefore, the delivery pressure of the gas is not constantly equal to the set pressure but departs from the latter according to the position of the shutter. The above mentioned direct action pressure regulator poses the further drawback that it does not allow the set pressure to be modified from a remote position.
In fact, for safety reasons, said setting is carried out by means of an adjusting screw located on the pressure regulator, which modifies the extent of deformation of the spring so as to vary the force it exerts, with the shutter in the same position.
Obviously, the fact that it is not possible to carry out the setting from a remote position leads to the further drawback that said setting must be made on site, with consequently higher labour costs.
A second known technique for controlling the pressure regulator includes an indirect feedback through a pilot device that varies the pressure in the motorisation chamber so that the delivery pressure of the natural gas is kept at the value set in the pilot device.
Prior art regulators operating according to the above second known technique are disclosed in U.S. Pat. Nos. 2,277,162 and in 2,042,781.
Compared to the pressure regulator system previously described, the one just described above offers the advantage that it allows a more precise regulation of the gas delivery pressure.
Still advantageously, the setting of the pilot device can be modified from a remote position, as the setting device has less stringent safety requirements than the pressure regulator.
However, the system just described above poses the drawback that its response time is longer than the response time of the first system, as the latter includes also the reaction times of the pilot device.
A further drawback posed by the system just described above lies in that a malfunction of the pilot device results in the impossibility of regulating the gas delivery pressure, which limits the reliability of the pressure regulator system.